1. Field of the Invention
The present invention relates to code division multiple access (CDMA) mobile communications systems, and in particular to a method of utilizing the downlink signal that is received by a mobile terminal in a CDMA mobile communications system.
2. Description of Related Art
In a CDMA mobile communications system, the transmission power used by base stations is controlled so that the power of the radio wave transmitted from base stations to mobile terminals is kept as low as possible, while maintaining constant channel quality.
A conventional transmission power control method with base station selection in a wideband CDMA mobile communications system will now be described with reference to FIG. 9, which is a conceptual diagram aiding a general explanation of such a transmission power control method.
A CDMA mobile communications system comprises a plurality of base stations and a plurality of mobile terminals. FIG. 9 shows two base stations 91 and 92, and one mobile terminal 93, of such a CDMA mobile communications system.
It is herein assumed that mobile terminal 93 is in the soft handover state and is communicating with base stations 91 and 92 simultaneously.
Mobile terminal 93 measures the propagation characteristics of the common pilot channel (CPICH) transmitted by the base stations and discerns which of the base stations gives the better common pilot channel propagation characteristics. Mobile terminal 93 then notifies the base stations of the ID of this base station.
The base station specified by the ID sent from mobile terminal 93 transmits downlink data to mobile terminal 93 using the dedicated physical data channel (DPDCH). The base station that has not been specified does not transmit downlink data to terminal 93.
Base stations 91 and 92 transmit control signals to mobile terminal 93 using the dedicated physical control channel (DPCCH), irrespective of whether or not the base station has been specified by the base station ID sent from mobile terminal 93.
Mobile terminal 93 transmits to base stations 91 and 92, in addition to a base station ID, a transmission power control (TPC) command that instructs an increase or decrease in the transmission power of the downlink dedicated physical channels (DPCCH and DPDCH).
Base stations 91 and 92 increase or decrease the transmission power of the downlink dedicated physical channels in accordance with the instructions given by the TPC command from mobile terminal 93.
FIG. 10 gives an example of the signal format of the dedicated physical channels in the downlink, while FIG. 11 gives an example of the signal format of the dedicated physical channels in the uplink.
In the downlink signal format illustrated in FIG. 10, the dedicated physical control channel and the dedicated physical data channel are time division multiplexed. Each slot is a fixed time length signal containing a field for the dedicated physical control channel and a field for the dedicated physical data channel.
In the uplink signal format illustrated in FIG. 11, the dedicated physical control channel and the dedicated physical data channel are quadrature modulated and multiplexed as mutually orthogonal signal components. Like the downlink slots, each uplink slot is a fixed time length signal.
A pilot signal, a feedback information (FBI) signal and a TPC signal are time multiplexed in the uplink dedicated physical control channels.
Mobile terminal 93 uses the FBI signal over a number of slots to notify base stations 91 and 92 of a base station ID.
FIG. 12 is a block diagram showing an example of the constitution of mobile terminal 93.
Mobile terminal 93 comprises antenna 121, circulator 122, base station selector 123, downlink reception quality measuring circuit 124, downlink TPC command decision circuit 125, multiplexer 126 and data demodulator 127.
Antenna 121 receives downlink signals from the base stations and transmits an uplink signal to base station 91 or 92.
Circulator 122 inserts the downlink signals received by antenna 121 into mobile terminal 93 and supplies antenna 121 with the uplink signal for transmission to the base station.
Base station selector 123 measures, for each base station, the propagation loss experienced by the downlink signal and selects, from the plurality of base stations, the one giving the smallest propagation loss.
Downlink reception quality measuring circuit 124 measures the reception quality of the downlink signal from the base station selected by base station selector 123.
On the basis of the reception quality measured by downlink reception quality measuring circuit 124, downlink TPC command decision circuit 125 decides whether the transmission power of the base station should be increased or decreased, and outputs the result of this decision as a TPC command. For example, downlink TPC command decision circuit 125 outputs a TPC command for instructing the base station to increase its transmission power when the reception quality is below a prescribed threshold, and to decrease its transmission power when the reception quality is above the threshold.
Multiplexer 126 multiplexes the uplink data that is input from terminal 128, the base station ID that is input from base station selector 123, and the TPC command that is input from downlink TPC command decision circuit 125. Multiplexer 126 outputs the multiplexed signal to circulator 122 as the uplink signal for transmission to the base station.
Data demodulator 127 demodulates the downlink data from the downlink signal of the base station that has been specified by base station selector 123, and outputs the demodulated data from terminal 129.
FIG. 13 is a block diagram showing an example of the constitution of downlink reception quality measuring circuit 124.
Downlink reception quality measuring circuit 124 comprises matched filter 131 and signal-to-interference ratio (SIR) measuring circuit 132.
Matched filter 131 demodulates the signal of the base station specified by base station selector 123.
SIR measuring circuit 132 measures and outputs the signal-to-interference power ratio of the output signal from matched filter 131.
FIG. 14 is a block diagram showing an example of the constitution of data demodulator 127.
Matched filter 141 demodulates the signal of the base station specified by base station selector 123.
Error-correcting decoder 142 performs error correction and decoding of the output signal of matched filter 141, thereby decoding the downlink data.
FIG. 15 serves to explain the transmission power control for the downlink dedicated physical channels.
In mobile terminal 93, downlink reception quality measuring circuit 124 uses the signal in the dedicated physical control channel (DPCCH) contained in the downlink signal received from the base station to measure reception quality, and uses downlink TPC command decision circuit 125 to decide which TPC command to give.
The TPC command is transmitted as a TPC signal in the uplink dedicated physical control channel.
The base station receives the TPC signal from mobile terminal 93 and, in accordance with the instruction given by the TPC command, either increases or decreases the transmission power of the downlink dedicated physical channel on a slot-by-slot basis.
FIG. 16 serves to explain transmission control by base station selection using base station ID. The explanation will be given in terms of the two base stations 91 and 92 and the mobile terminal 93 depicted in FIG. 9.
Firstly, mobile terminal 93 uses a plurality of FBI signals in the uplink dedicated physical control channel to notify base stations 91 and 92 of the base station ID selected by base station selector 123. The base station ID is encoded to make it less susceptible to transmission path error.
When base stations 91 and 92 receive the information contained in the final slot employed to convey the base station ID, they decide, on the basis of the received base station ID, whether or not to transmit over the dedicated physical data channel (DPDCH) after a prescribed number of slots. Namely, the base station specified by the received base station ID (in the example illustrated in FIG. 16, base station 92) transmits over the DPDCH, while the base station that has not been specified (in this case, base station 91) does not transmit over the DPDCH.
However, for both base stations 91 and 92, the decision not to transmit the DPDCH can only be made when the reception quality of the signal bearing the base station ID is sufficiently good. This makes it possible to avoid the situation where, due to transmission error, neither of the base stations transmits the DPDCH.
Base stations 91 and 92 always transmit the dedicated physical control channel (DPCCH) regardless of which base station ID is specified. Consequently, although the DPCCH may be received from a number of base stations, mobile terminal 93 decides which TPC command to send by looking at the DPCCH received from the base station that is transmitting in the DPDCH. Because a TPC command serves to control transmission power in the DPDCH, it is necessary to look at the DPCCH from the base station that is transmitting the DPDCH.
However, the following problem is encountered in a conventional transmission power control method with base station selection.
Namely, if the propagation environment for the uplink signal is poor and a base station cannot receive the base station ID from mobile terminal 93 correctly, a base station other than the one specified by mobile terminal 93 may transmit downlink data. If this occurs, the downlink data from the unintended base station constitutes an interfering wave with respect to the other downlink signal.
Data demodulator 127 of mobile terminal 93 demodulates downlink data using only the downlink signal received from the base station that was specified in terms of its base station ID, in other words, using only the downlink signal received from the base station that was instructed to transmit downlink data. Consequently, an error in the transmission of the base station ID will result in none of the downlink data transmitted from unintended base stations being utilized, and hence in a decrease in frequency utilization efficiency.